Hello again, last thread didn't get much attention so I update my problem and post a new thread. Yes I've tried every obvious thing and I will now paste my letter to Nintendo. Help is unbelievably appreciated, you can get a free shiny or whatever from my tradeshop if your help is what solves the problem.

My letter to Nintendo: Hello Nintendo Support!

I have already tried talking to the guy working at Nintendo Support Sweden but he had no idea what could be wrong etc.
Here's my issue: 1-2 months ago I could trade over WiFi flawlessly on my Pokemon White on my 3ds. Now when I picked up my trading hobby I have been unable to trade with 3/4 people I've attempted with. One of us (usually the other person) gets booted out with the error code 86420, I have checked up your help page but I'm not positive to turn of my firewall, it seems very risky. And since I was able to trade before I think the problem lies elsewhere. See, I can browse the internet, download from the eShop, download Pokemon Black and White events etc. However, I simply can not make a connection with another person (yes, we have checked our Pal Pads multiple times). I'm 99% sure the problem is on my side because the others can trade with whoever they want except for me. My own theory is that some firewall is blocking some connections because it's only connecting to other people that doesn't work. However my knowledge of internet issues are very low...

I have tried: Resetting my router. Trading with other people (1 out of 4 worked). Setting my IP adress, Subnet mask, Gateway and DNS values to specific numbers seen on my router control panel. Deleting and re-entering my Internet settings multiple times. Trying the same thing on other Pokemon cartridges.

That's all I can remember now, none of these things solved my problem. Now, the only thing that has changed between being able to connect/trade and this is that the router is now connected to a new computer (does the computers firewall interfere with the security between the router and my 3ds?). I repeat, it's is ONLY trying to trade/battle with friends over wifi that does not work, I can do everything else perfectly using internet on my 3ds.

My last resort is this:
Open ports 1 through 65535 as both TCP and UDP.
Ensure the ports are forwarded to the same IP Address given to the Nintendo DS system during the manual setup (info from your website)

Now, I tried making a "port forward" at my router control panel, entering the numbers 1 through 65535 and then just "add". When trying to assign this port to my 3ds device it didn't work. There was 2 blank spots for more text to enter when making the port, I did not know what to put there. And "Ensure the ports are forwarded to the same IP Address given to the Nintendo DS system during the manual setup " is not understandable for me, where do I put in the IP adress for my ports? I attached a screenshot of how it looks when I'm creating a port."

And now I have also tried turning of the firewall on both Router and Computer, entering the exact correct IP adress etc and also tried opening port 80 properly. I've "Power-Cycled" my router by unplugging everything and let it rest 5-6 minutes. I won't buy a new router because it worked before and I "just" need to figure out the damn issue. Thanks!